1. Field of the Invention
The present invention relates to a light-weight composite material having a low density, good strength, high modulus of elasticity and rigidity, with good homogeneity as regards these properties. It concerns a composite material suitable in use for reinforcing materials or structural materials for an airplane, a land vehicle such as a car, sounding plates of a marimba or a xylophone and a sounding board of a piano or a guitar, a diaphragm of a speaker, and a sports article such as a surfboard, a racket for pingpong, a table for pingpong playing, a ski, etc.
2. Description of the Prior Art
As a conventional light-weight composite material for use as a structural material for musical instruments etc., materials disclosed in JP-A-49-127612, JP-A-55-9504, JP-A-56-99656 and JP-A-56-162644 are known. Each of such conventional materials is made as a composite of a resin and reinforcing fibers. As the resin which constitutes a matrix of the material has independent cells caused by foaming, the material has the properties of light weight as well as good strength and high modulus of elasticity. However, the material is not so good on homogeneity, in respect of a large variation of strength, or modulus of elasticity or rigidity throughout the material. This defect of the material is due to a considerable non-uniformity in the distribution and size of the independent cells.
This non-uniformity of the distribution and size of the independent cells arises mainly during the manufacturing process of the above material. In the process, the resin containing foaming agent and reinforcing fibers is compounded, and the foaming and forming of the composite material are concurrently performed by heating the material. Since the cells which are generated in the material during the process can move easily along the axial directions of the fibers but cannot easily move in other directions, considerable non-uniformity of the distribution of the cells is liable to occur. Also, since some cells grow by coalescing with each other, there occur great differences between the sizes of such expanded cells and the sizes of uncoalesced cells, thereby causing a large disparity in the sizes of the cells.
Moreover, in the above conventional material, when the expansion ratio of foaming is increased in order to decrease the density of the material to a low value, the coalescence of the cells occurs violently and long or widely continuous cells (open cells) are generated. The open cells cause the strength, modulus of elasticity and rigidity of the material to decrease. Therefore, the density of the material cannot be reduced too much, the possible density being, at lowest, about 0.9 g/cm.sup.3.
Furthermore, as the matrix resin of the above material is hardened during foaming, it is difficult to control the timing of the hardening and the temperature distribution in the thickness direction of the material. Accordingly, it becomes difficult to control the expansion ratio of foaming and the porosity of the material, and consequently to obtain a material with a proper balance of strength, modulus of elasticity and rigidity.
On the other hand, JP-B-47-21754, JP-B-49-29448 and JP-A-51-94911 disclose materials in which a foamed plastic or a low specific gravity plastic (for example, styrol foam) is sandwiched between two fiber reinforced plastic layers. In such materials, however, as the core layer of the material is constructed of merely a porous layer of a plastic, the strength, modulus of elasticity and rigidity of the layer itself can not be increased much. As a result, the mechanical properties of the material as a whole are not very high. To give the material a strength etc. higher than a certain level, it is necessary to increase the thickness of the material, and so it is difficult to obtain a relatively thin material. Moreover, as there is a great difference of modulus of elasticity between the core layer and the outer layers of the material, the sound effect of the material is not very good when it is applied to a musical instrument or a speaker.
Furthermore, there is a known composite material in which a layer of glass fiber mat or cellulose fiber mat is sandwiched between outer fiber reinforced plastic layers. In such a material, however, as the glass fiber mat or the cellulose fiber mat constituting a core layer is conventional, the strength, and modulus of elasticity and rigidity of the core layer are extremely low. Therefore, the mechanical properties of the composite material as a whole are also low. In order to give a high strength to the material, the thickness ratio of the outer fiber reinforced plastic layers must be increased, but, if this is done, the material will not be light in weight.
Still furthermore, there is a known composite material which has a structure in which a layer of a honeycomb-construction aluminium or paper is sandwiched between outer fiber reinforced plastic layers. In such a material, however, as the core layer having a honeycomb-construction must be of a certain thickness, there is a limitation on forming a thin composite material. Also, it is difficult to freely set or change the ratio between the thickness of the core layer and the thickness of the outer layers.